This invention relates to signal processing of time domain electronic signals, such as video information signals. More particularly, the invention relates to techniques for encrypting and decrypting such signals to prevent unauthorized use thereof.
Many techniques have been devised for encrypting and decrypting time domain information signals. The purpose for such techniques is always the same: viz., to prevent unauthorized use of the signals. In the case of video type information signals, the unauthorized use to be prevented is normally the visual display of the information signals for their entertainment or instructional value. Such signals are vulnerable to unauthorized use in a number of ways. For example, if the video signals are being broadcast over a satellite or microwave link, unauthorized users attempt to intercept the signals and view same without paying for the subscription service. In an effort to defeat such unauthorized uses of broadcast video information, several specific signal scrambling techniques have been successfully used.
Another means of conveying video information from one location to another is through the medium of video tape. For example, it is quite common for motion picture studios to send master videotapes of movies around the world. If the videotapes get stolen or "lost" in transit, a clear opportunity for piracy exists. It is therefore desirable to be able to scramble the video signal prior to recording it on videotape so that the tape can only be utilized by a user having a descrambler and appropriate codes. Such a scrambling system must have two important characteristics--it must be very secure and it must be compatible with the record/replay electronics of preferably all professional and consumer grade video recorders.
There are may known ways for scrambling video signals. Two simple techniques are sync suppression and sync inversion, each of which can, however, be readily defeated by using elementary video signal processing techniques and in any case cannot be recorded. Another technique is termed pseudo-random video-level inversion, which is relatively difficult to defeat but which suffers from the disadvantage of a severe loss of picture quality due to non-linearities in the record/playback process. Still another technique is line-order interchange, also known as line shuffling, in which the order of the lines in the raster scanned picture is shuffled. As an example, instead of transmitting the lines sequentially as line number 1, line number 2, line number 3, . . . etc., the information might be transmitted as line number 182, line number 99, line number 4 . . . , etc. Such a system can be made very secure (i.e., very difficult to defeat), but it cannot be used in any videotape format employing the color-under principle which relies upon line adjacency to obtain correct color rendition upon reproduction.
Still another technique is pseudo-random line rotation in which some of the lines of the picture selected in random fashion are transmitted in inverse temporal order (i.e., right to left), while the remainder are transmitted in the normal fashion (i.e., left to right). Yet another technique is termed line segmentation with pseudo-randomly chosen break points, in which each line is broken into two randomly chosen segments and the segments are sequentially transmitted with the right hand segment being transmitted first, followed by the left hand segment. Both of these video signal scrambling methods give rise to severe color contamination between the left and right hand sides of the picture when employed on any format which uses color-under recording.
In addition to the above disadvantages, the last three noted techniques suffer from the further disadvantage that the processing is incompatible with the drop out compensation signal processing employed in most video recorder devices. While, in principle, these three methods could be used for video signal processing formats which do not employ color-under recording, such as professional type B and type C one inch formats, such a use would require special drop out compensation circuitry in which drop out sensing and correction are controlled by the descrambling system. This would require special modification of playback equipment, which adds undesired cost and complexity to an encryption/decryption system.
None of the above-described video scrambling techniques fully meets the desired requirements for a video scrambling system in which (1) the scrambled video can be recorded and subsequently replayed on any video tape format--professional or consumer--and be descrambled on replay, with negligible loss of picture quality; (2) the scrambling technique is virtually impossible to defeat by any unauthorized user; and (3) the scrambled video is unaffected by passage through the various kinds of processing equipment used in television production facilities, satellite links and cable networks.
In the above-referenced copending U.S. Patent Application Ser. No. 203,676, now U.S. Pat. No. 4,916,736 a method and apparatus are disclosed which provides a highly secure video type information signal encryption and decryption technique which is compatible with all video tape formats and transmission systems and is free of picture impairments caused by the interaction of the scrambling algorithm and the chrominance consecutive--line averaging--system used in color-heterodyne recording. According to the method disclosed therein, video type information signals are encrypted by individually time shifting the active video portion of at least some of the lines of the video signals with respect to the line timing reference (horizontal sync in an NTSC encoded system) and providing an indication of the time shifting performed in order to enable subsequent decryption. For color video information signals, time shifting is inhibited during the horizontal sync signal portion and the color reference signal portion. Similarly, the non-active video portions of a field or frame of information (i.e., the vertical blanking portions) are not time shifted. Decryption of the encrypted signals is accomplished by using a process which is the inverse of the encryption process. For optimum results, and in order to ensure compatibility between the encryption method and other conventional signal processing techniques (in particular the color-heterdodyne system of video cassette recorders) the amount of time shift between adjacent lines is preferably limited to .+-.N subcarrier cycles where N is a whole number (preferably 0 or 1). In addition, the maximum aggregate time shift of the active video is limited so that the active video does not overlap either the color burst or the horizontal sync reference portions of the individual lines.
While the above encryption/decryption technique is highly effective, optimal implementation requires digital video circuitry at the decryption site (i.e., the t.v. monitor or receiver) of some complexity, which adds substantial cost to the total system.